X-ray beams thin out
Jul 11, 2002
X-rays scattering is widely used to probe the structure of solids, but the technique is limited by the ‘spot size’ of the beam. The smallest available spot size is currently several hundred nanometres across, but now Tim Salditt of Saarlandes University in Germany and co-workers have created a coherent X-ray beam that is ten times narrower. Based on waveguides, the sources could be used to examine single nanocrystals or arrays of molecules in biological cells (F Pfeiffer et al 2002 Science 297 230).
When X-rays are shone into a crystal, they are reflected at certain angles by the regular planes of atoms in the crystal. This means that the structure of the crystal can be deduced by studying the reflected X-rays. Since it is difficult to prepare large, good quality crystals, samples are often ground into a powder. In this case, the intensity of the reflected X-rays is recorded as a function of reflection angle, with the peaks in this spectrum showing the dominant angles of reflection.
But this technique only works if the sample is uniform over an area that is at least as large as the cross-section of the X-ray beam. ‘Pinholes’ are used in some analytical techniques to create beams that are micrometres across, but smaller pinholes – even if they could be made – would not transmit a sufficiently high flux of X-rays.
Salditt’s team has solved this problem by making an X-ray waveguide with an exit aperture tens of nanometres across, which emits a beam with an oval cross-section just 69 nm wide and 33 nm high. The researchers say that this is the smallest spot size ever achieved for short-wavelength – or ‘hard’ – X-rays.
In common with optical devices, the waveguide consists of a core with a higher refractive index than the cladding layer that surrounds it. The X-rays travel through the waveguide as a series of reflections that take place each time the beam reaches the boundary between these layers. By adjusting the angle at which the X-rays entered the waveguide, the researchers could select the modes of propagation that travelled through the guide. This allowed them to generate coherent beams of X-rays using the waveguide.
Salditt’s team used a recently developed technique known as ‘resonant coupling’ to shine the X-rays into the waveguide. The core of the waveguide was made from poly(methyl methacrylate) core and the cladding was made from chromium, but this choice of materials was determined by the lithographic technique they used to make the waveguide. With further refinements, they hope to make a waveguide that will produce even narrower beams.
About the author
Katie Pennicott is Editor of PhysicsWeb